This invention relates to electrodeposition coating compositions containing a water-soluble or water-dispersible modified epoxy resin as an electrically depositable film-forming component thereof.
Electrically depositable, film-forming resins used in anodic electrodeposition coating compositions generally have a number of hydrophilic groups such as carboxylic group for rendering the resins negatively chargeable and water-soluble or water-dispersible. A relatively large number of such hydrophilic groups must be introduced to the resin molecule for imparting a desired level of water-solubility or water-dispersibility. This, in turn, requires the use of a large amount of base for neutrallizing acid groups possessed by the resin. Also, the resins tend to be liable to hydrolysis and thus are unstable upon storage. The introduction of a large number of hydrophilic groups results in a decreased Coulomb efficiency and throwing power during the electrodeposition coating process while consuming a large quantity of electric energy.
Typical examples of resins used in the anodic electrodeposition coating process include maleinized drying oils, maleinized liquid polybutadiene, acrylic polymers having acid groups and the like. These resins themselves are not satisfactory in their film properties such as corrosion resistance or adhesion strength to substrates. In order to overcome these defects, water-insoluble thermosetting resins such as epoxy acrylate resins are often incorporated to anodic coating compositions as an aqueous emulsion. Since the base resins themselves do not function as a surfactant, they must be emulsified by means of a separate emulsifier which may adversely affect the film properties such as water resistance.
Epoxy resins are known to have many advantageous characteristics such as good adhesion to metal substrates, high corrosion and chemical resistance, high electrical insulating property and the like. For this reason their use is ever increasing in a wide variety of fields. Generally, epoxy resins are used as such or as a solution in an organic solvent. It is often difficult to obtain an optimal balance between their hardness and other properties when used in coating compositions. Water-based epoxy coating compositions may be produced either by emulsifying water-insoluble resins or by introducing hydrophilic moieties into the resin backbone to make them water-soluble or water-dispersible. The resulting coating compositions, however, must suffer from the above described disadvantages such as decrease in water-resistance, storage stability and other properties.
It is known in the prior art to produce a modified epoxy resin having a plurality of flexible side chains by graft polymerizing .epsilon.-caprolactone to alcoholic hydroxy groups in the middle of an epoxy resin molecule. The resin also has a plurality of primary alcoholic hydroxy groups at the terminal of each side chain. See, Japanese Laid Open (Kokai) Patent Application Nos. 164116/1982, 187463/1983 and 44915/1986. However, resins of this type do not possess any chargeable group required for electrically depositable resins.